I first heard of the expression in a fancy dinner a relative brought a diplomat she was dating, and for diplomats all around the world for the last four or five centuries French was the Lingua Franca, as well for meetings between courts, aristocrats, academics, and the first language a book would be translated to because up until a few generations ago if one wanted to learn an "international language" it would learn French, which makes the etymological assumption understandable. However I just went to check some info about it since I was just told Franca doesn't come from French... turns out, it has nothing to do with Germanic languages as well, but a mix of mostly Romance languages used through the Mediterranean for trade and that maybe got its name because Arabs, Turks, Persians, etc called all Europeans "Franks" and it was synonym of Westerner.